OCEANOLOGICA ACTA. 1990. VOL. SPÉCIAL 10 ~ ---~~ Microplate Eastcr island A tectonic test Plate kinematics Rap.ll1ui expcdition Ridge tcctonics of instantaneous kinematics of the Mic roplaquc Ile de Pâques Cinématiqu~.des plaquel! Expe<1mon Rapam.n Easter microplate Tcclomque des dorsales Janet H. ZUKIN, Jean FRANCHETEAU Inslitut de Physique du Globe, 4 Place Jussieu, 75252 Paris Cedex 05, France. Received 03/11/89, in rcviscd fonn 09/04/90, accepted 1O/O5!90. ABSTRACT Oetailed studies of the Easler microplate, and other contemporary microplates located along the world's mid-oceanic ridge system, are considered of primary importance to our understanding of the kinematic evolution of the major oceanic plates. The focus of the Rapanu i expedition, pan of the "Tour du monde" of the French research vessel Jean CluJrcol, was to collee! detailed structural and geophysical data along the nonhem and southem Easter microplate boundaries in order to test, in a rigorous manner, the most recent tectonic and kinematic models for Ihe micro plaie. ln developing detailed bathymetric, morphoteclonic and magnelic maps of Ihese boundaries, we have found lhal our new plate boundaries differ considerably from Ihose of Hey et al. (1985). These new boundaries arc characterized by complex defonnational structures, particularly at Ihe northern and soulhern triple junctions. The norlhern boundary is dominated by compression and right lateral shear, paftÎCularly to the west, and leaùs 10 extension in the east. The southem boundary is dominaled by extension, particularly to the west, lcading to right lateral shear and compression 10 the east. We have utilized our detailed data to test Naar and Hey's (1989) instantaneous Euler poles for the Nazca-Easter and Pacific-Easter plate pain; in both a qualitative and quantitative sense, first by comparing the locations of differing defonnational regimes (predicted by the Euler pole locations and the locations of the limits between these regimes) with our new plate boundary configurations, and secondly, by making precise comparisons between our profiles and magnetic models which we calculated using the pole locations and angular rotations. We have found that our data agree fairly weil with the Nazca-Easter and Pacific-Easter instantaneous Eu ler pole positions. We have also tested the kinematic microplate models of Engeln et al. (1988) and Schouten et al. (1988a, 1988b), and have shown Ihal our data are in general agreement with their model of ri gid plaie rotation, but not with their model of simple shear. Oceanologica Acta, 1990, volume spécial 10, Actes du colloque Tour du Monde Jean Charcot, 2-3 man; 1989, Paris. 000-000. RÉsuMÉ Test tectonique de la cinématique instantanée de la microplaque de l'île de Pâques Les études détaillées de la microplaque dc l'îl e dc Pâques, et d'autres microplaques présentes le long du système des dorsales médio-océaniques son t essentielles pour comprendre l'évolution cinématique des plaquer. océaniques majeures. L'objectif principal de l'expédition Rapanui, dans le cadre du Tour du Monde du NID Jean Charcot était d'acquérir des données structurales et géophysiq ues détaillées le long des frontières Nord et Sud de la microplaque de l'île de Pâques afin de tester les modèles cinématiques ct tectoniques les plus récent s de la microplaque. La cartographie bathymétrique, 183 J. H. ZUKIN, J. FRANCHl:.iEAU morphotectoniq ue et magnétique a montré que les frontières de la microplaque dilRrent beaucoup de celles proposées par Hey el al. (1985). Ces fronti ères nouve llement définies som marquées par des structures à déformation complexe, en pa rt iculier aux points triples Nord ct Sud de la microplaque. La frontière Nord est caractérisée par une compression ct un cisai llement dextré, surtout à rOuest, qui passe à une extension li l'Est. La frOll1ière Sud est caractéri sée par une extension. surtout à l'Ouest. qui passe à un cisai ll ement dextre et une compression à l'Est. Les données détaillées ont permis de tester les pôles d'Euler instantanés du modèle de Naar et Hey ( 1989) pour les mouvements Nazca-Pâques et Pacifique-Pâques sur le plan qualitatif ct quantilif d'abord en comparant les do mai nes des différents régimes de déformation (prédits par les pôles d'Eu ler) le long des nouvelles frontières et ensuite en comparant les profi ls magnét iques aux pro fil s synthétiques prédits par les paramètres de rotalÎon. Les données sont en accord raisonnable avec les pôles eulériens instantanés qui déc ri vent les mouvemenls relatifs Nazca-Pacifique et Pacifique­ Pâques. Nous avons également testé les modèles ci nématiques de microplaque d'Engeln el al. ( 1988) cl Schouten el al. (1988a, 1988b) et montrons que les données sont en accord avec leur modèle de rotation de plaques rig ides mais en désaccord avec leur modèle de cisai llement simple. OceanofogÎca Acta. 1990, volume spécial 10, Actes du colloque Tour du Monde Jean Charcot, 2-3 mars 1989, Paris. 000-000. INTRODUCTION The focus of the Rapanui expedition, which took place in carly 1987 and was part of the "Tour du Monde" of the French research vesseJ NID Jean Charcot, was 10 colleet detailed structural and geophysieal data along the During the last decade of rcseareh over the world's mid­ northern and southern Easler mieroplate boundaries: ocean ridge system, scientists have come to reahze tha! these areas are depicted as areas 1 and 2 in Figure 1. thi s system is made up of a series of complex With these detailed dala (comprising Seabeam swalh overlapping rift discontinuities. The largest of Ihese, balhymelry (Renard and Alienou, 1979), magneties and mÎcroplates, have been id enlified bath along the gravit y) wc ai med to test, in a ri gorous way, the prev ious eontemporary mid-ocean ridge system as weil as fou nd plate boundary configuration (Bey el al., 1985) and the "frozen" into the old ocean basins (Mammerickx et al., mosl rceent kinemalic models for the microplate (Naar 1988) and are thought to play a primary role in the and Hey, 1989; Engeln et al., 19 88; Se hou len el af., kinematie reorganization of major oceanic plates. Siudies 1988). The preliminary res ults from Ihis cxpcdition were wh ich focus o n the kinematie evol ution of these presented in a paper by Franeheleau el al. (1988). ln mieroplates arc thercfore considercd important 10 our another paper, Zu kin and Franeheteau ( 1990) have understanding of the more gencral evolution of the major combincd Seabeam data from the Rapanu i expedition oceanie plates, and perhaps 10 the geodynamic processcs with five other expcditi ons (American and Gemlan) and controlling this evolution. have made a detailcd assessment of the tectonies and plate boundaries in the aceas depicted in Figure 1. In this The Easter mieroplale , localed bctwccn 22°S and 28°S paper wc shall briefly show the results from our previous along the East Pacifie Rise (EPR) (Fig. 1), is the OOSl work, and then discuss how they have been uscd to test known of the eontemporary mid-ocean ridgc microplatcs. the location of the Pacifie. Nazca. and Easter The majority of studics here have been reconnaissance in instantaneous Euler poles (Naar and Hey, 1989) and the nature and have foeused on redefining the microplatc kinematie microplate models rccenlly devised by Engeln boundaries (Herron 1972, Forsyth 1972, Anderson el al. et al. ( 1988) and Schouten et al. (1988a and b). 1974, Handschumacher et al. 1981, Engeln and Stein 1984, Engeln 1985, and Hey et al. , 1985). Dther studies have focused on the ki nemati c evolutioll of the mieroplate. some by invening inslantaneous Euler poles MORPHOTECTONIC MAPS AND PLATE of rotai ion for the Pacific-Easter, Nazca-Eastcr, and BOUN DARY CONFIGURATIONS Nazca-Pacific plate pairs (Engcl n and Stein 1984: Naar and Hey, 1989). and Olhers by produeing rigid and shcar behaviour models for mieroplates in general, and the ln order 10 make a delailed nssessment of the tectonies Easter microplale in partieular (Engeln et al. 19 88: along the northcrn and southern bound aries of the Schoulen et al. 1988). More recent cxpcditions tO th is mi cropl ate. Zu kin and Francheteau (1990) prepared a arca have rcsulted in an almosl IOtal side-scan sonar se ri es of 200m contour interval bathymetric maps and a insonification of the microplale boundari es and interior series of rnorphoteclOnîe maps (Figs 2 and 3) which (Hey and Naar. 1987; Searle et al .. 1989 ). integrated Ihese bathymctrîc data with other geological 184 TEcroNIC lEST OF EASTER MICROPLATE KINEMATICS 118°W 115°W 109"W 22 ·S~~1 ---'------------~~~lrï'~:r1 ... ~~--~~=c======~~----r---~"l2• • 2 0S r-".'---··-----·7.----' • Area 1 • , " , . , • ,0 f~,,-. o 0 __ ~ + + + ' , t _< "- t oi - . ---;--- ""oœ~.~'\.\\ + + \~', 0 \1 (" 0 ' .. ~ o "0 • 'oH HAl C • + .. + + + + + • • • . " ". '1.II\ "'" EASTEA .. 'CAOI'L' .n Il o II · '. "" +- .~\: + + + 'l" +: + + + : \r~' ~~- : '. +~--+-- i \~ : . \-:;,:.: ' ••• M~a 2 • , 1 + '. ..... • + + + • • + .... e~M" ,+' 0 + .", ,,,,, . <!. • o 0 :l- • • e " , e o : , .. , •• •• , e • .. L ' . • :+ -;/1 + • + + .. .#' "t-_, : • .... "',~ ." 30 o S,~c_----"-------__----_cc"oc-----"---- ---L------"·"-"·L----"-------""-----~ 30"5 118° W 115°W 10goW Figure 1 ûxalion ofllre Easler Microplate in the SE Pacifie (imet). Boundories of the Easftr microplaft: Francheteau et al. (1988), Naar and Hey (1986) and Hey et al. (1985). Double linl!S: spreading Un/ft!. Single line: Iransform fau/t. DOlfed IiMS: pseuJofaults of Hey et al. (/985). Dors: eanhquakt epicentres, from Ihe World-Wide StandorJizeJ Seismographic Network (WWSSN). Boxl!S QU.tline rire two survey areas discussed in rhe lexl. Mercator projection. Position de la mieroplaque de lile de Pâques dans le Sud-Est PacifIque (canooche). Frontières de la microplaque : Franeheteau tl al, (1988), Nue et Hey (1 986) et Hey el al.